1. Field of the Invention:
This invention relates to an anti-skid control system for motor vehicles, which is adapted to prevent the wheels of the motor vehicle from skidding during braking operation, and more particularly it pertains to such a system designed to prevent a brake pressure reduction mode from occurring or to immediately release such mode even if it occurred, when the motor vehicle is stopped, thereby preventing the vehicle from sliding down when stopped on a sloping road surface or the like.
2. Description of the Prior Art:
Generally, with an anti-skid control system for motor vehicles, braking control is effected with the aid of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrates the relationships between wheel speed Vw, brake pressure Pw, and decay signal for opening decay valves so that the brake pressure is reduced, which occur during the operation of the above-mentioned conventional anti-skid control device.
When the brake pressure Pw is not increased while the motor vehicle is running, the hold valves remain opened while the decay valves remain closed, and under such a condition, if the brake pressure Pw is increased, then the wheel speed Vw is decreased. When the deceleration (negative acceleration) of the wheel reaches a predetermined level, the hold valves are opened so that the brake pressure is held, and subsequently the wheel speed Vw is abruptly decreased. At time when the wheel speed Vw becomes equal to a reference wheel speed Vt which is previously set up on the basis of the wheel speed Vw in such a manner that it follows the wheel speed Vw with a predetermined relationship thereto, i.e., at time t1 when the curve representing the wheel speed Vw and that representing the reference wheel speed Vt cross each other, the decay signal is generated so that reduction of the brake pressure Pw is started. As a result of this brake pressure reduction, the wheel speed is changed from deceleration to acceleration at time 2 when a low peak V1 of the wheel speed occurs; at time t3 when the wheel speed Vw recovers up to a speed Vb which is higher than the low peak V1 by 15% of the difference A between the wheel speed Va at the brake pressure reduction starting time t1 and the wheel speed V1 at the low-peak point t2 (Vb=V1+0.15 A), the brake pressure is held; and subsequently, at time t4 when a high peak Vh of the wheel speed Vw is reached, buildup of the brake pressure is started again. The buildup of the brake pressure Pw is effected in such a manner that the brake pressure is increased and held alternately and mincingly in succession so that the brake pressure Pw is caused to build up gradually. At time when the wheel speed Vw becomes equal to the reference wheel speed Vt, i.e., at time t5 when the curve representing the wheel speed Vw and that representing the reference wheel speed Vt cross each other, the next brake pressure reduction cycle is performed. Repetition of such control cycle results in the vehicle speed being decreased without wheel skidding.
The above anti-skid control is effected by means of a microcomputer on the basis of electrical signals derived from wheel speed sensors adapted to detect the respective wheel speeds, and there is a minimum speed Vmin detectable by the central processing unit (CPU) of the microcomputer. With the conventional anti-skid control procedures, when the wheel speed Vw goes below the minimum speed Vmin, say 4 km/h, it is regarded as zero as shown by a solid line. Such a wheel speed Vs detected by the CPU will be referred to as "system speed" hereinafter. The system speed Vs coincides with the wheel speed Vw except when the wheel speed is equal to or lower than the minimum speed Vmin. At time when the system speed Vs and reference wheel speed Vt become equal to each other, the decay signal is generated so that reduction of the brake pressure is started.
With the foregoing anti-skid control, when the vehicle runs on a road surface with a low friction coefficient u (low-u road surface), there is the tendency that the wheel speed is behind in recovery after the brake pressure is reduced; thus, a longer time is take before the wheel speed Vw reaches a high peak which is the next pressure buildup starting point, or in some cases no high peak of the wheel speed occurs at all, as a result of which a non-braking condition persists all this time. To avoid persistence of such a non-braking condition, it has been the practice that a timer is provided which is adapted, when no brake pressure reduction stopping point is reached after a lapse of a predetermined time period, for forcibly interrupting the hold signal HS to open the hold valves, thereby causing the brake pressure Pw to be increased abruptly.
Assume that at the time t1 when the system speed Vs is higher than the minimum speed Vmin, the decay signal is generated, by which reduction of the brake pressure is started. In such a case, since the wheel speed Vw is higher than the minimum speed Vmin at the time t1, the wheel speed Vw is prevented from dropping down to zero and permitted to recover through the low peak which occurs at the time t2, even if the wheel speed Vw is further decreased to go below the minimum speed Vmin; and at time t3 when the wheel speed Vw recovers up to the level of the minimum speed Vmin, the decay signal is interrupted. Thus, the brake pressure Pw is switched from reduction to holding at the time t3, and again increased at time t4 when high peak of the wheel speed Vw occurs.
At time t5 when the wheel speed Vw is decreased down to the level of the minimum speed Vmin, the system speed Vs becomes zero; thus, at the time t5, the curve representing the system speed Vs and that representing the reference wheel speed Vt cross each other so that the decay signal is generated. However, since the wheel speed Vw is decreased down to the level of the minimum speed Vmin, even if reduction of the brake pressure Pw is started, the wheel speed Vw is no longer permitted to recover so that the vehicle is stopped. For this reason, the decay signal is prevented from being interrupted based on recovery of the wheel speed, and thus it is caused to persist until it is forcibly interrupted by a timer which is started at the time t5 when the decay signal is generated; as a result, there occurs a non-braking condition which persists all this while. Assuming that the time set on the timer is 1.44 seconds, the non-braking condition persists for about 2 seconds including the period, say 0.6 seconds during which the brake pressure is held after the decay signal is interrupted. As a consequence, the braking distance will be increased, and thus there is the tendency that when stopped on a sloping road or the like, the motor vehicle tends to slide down.